Flexibility in relation to the human body can be defined as the range of motion (ROM) of a joint that is largely affected by the muscles, tendons, and bones around the joint (Borges, Medeiros, Minotto, Lima, 2017). There are several methods to increase ones flexibility and ROM. Proprioceptive neuromuscular facilitation (PNF) in particular is a stretching technique that has been shown to effectively increase ROM and flexibility (Hindle, Whitcomb, Briggs, Hong, 2012). PNF stretching can be performed to increase passive range of motion (PROM) and active range of motion (AROM). Two of the most common methods discussed in the current literature of PNF stretching include a contract relax (CR) method and a contract relax antagonist contract (CRAC) method. The CR method is performed by lengthening the muscle targeted for stretch and holding it in lengthened position while maximal isometric contracting of the same target muscle is being performed for a set amount of time. This is then followed by relaxation of the target muscle while being passively stretched. (Hindle et al., 2012; Maharjan, Mallikarjunaiah, 2015; Muscolino, 2017). The CRAC method of PNF stretching is performed similarly to CR but with an antagonist contraction instead of passive stretching following relaxation of the targeted muscle. There are four theories as to why PNF stretching is effective in increasing ROM. These include autogenic inhibition, reciprocal inhibition, stress relaxation, and the gate control theory (Hindle et al., 2012; Maharjan, Mallikarjunaiah, 2015).
Muscle spindles and Golgi tendon organs (GTO) are two types of muscle proprioceptors that are protective in nature but also play an important role in how these proposed mechanisms work to increase ROM during PNF stretching. Muscle spindles are located within the belly of a muscle and senses stretch or lengthening of a muscle (Powers & Howley, 2018; Muscolino, 2017). When a muscle becomes stretched (lengthened) to a point the muscle spindle too is stretched. The stretching of the muscle spindle causes an impulse and an afferent neuron is sent to the central nervous system (CNS) through the spinal cord. The CNS receives and interprets this information. If a muscle is lengthened too far the CNS will send an efferent neuron to cause a reflex contraction called a myotatic reflex to contract (shortening of the muscle) to prevent any more lengthening to that muscle to prevent damage or tearing. (Powers & Howley, 2018; Muscolino, 2017). The GTO is another type of muscle proprioceptor that is located near the musculotendinous junction and is attached to muscle fibers. The GTO detects changes in tension within a muscle. When a muscle contracts (shortens) increased tension is placed upon the GTO (Hindle et al., 2012; Powers & Howley, 2018; Maharjan, Mallikarjunaiah, 2015; Muscolino, 2017). The shortening of the muscle causes the GTO to become stretched and in turn creates an impulse that sends an afferent neuron to the CNS by way of the spinal cord. The CNS then interprets this information sent by the GTO detecting a pulling force on a tendon. This pulling force can damage and injure the tendon. This ultimately causes another impulse to be sent to relax the muscle so that no more tension is place upon the tendon (Powers & Howley, 2018; Muscolino, 2017) This is termed the GTO reflex or inverse myotatic reflex as it has the opposite or inverse effect of the myotatic reflex created by the muscle spindle.
It is important to note that both muscle proprioceptors discussed are protective in their design. Due to this protective element, the GTO’s inhibitory type reflex in particular can be utilized in increasing the amount of stretch that can be placed upon a muscle. Isometric contraction of the target muscle causes tension to be placed upon the muscle and its tendon, which in turn activates the GTO reflex. The GTO reflex causes relaxation of the targeted muscle due to its protective nature. This relaxation of the muscle by way of the GTO reflex prevents excessive tension or stretching on the tendon to avoid tearing or damage. This relaxation of the muscle allows for further stretch to be placed upon the targeted muscle and in turn allows for increases in ROM and ultimately flexibility seen from PNF stretching.
While by design the GTO acts as a protective measure by sending an inhibitory reflex to the target muscle, we are essentially using this reflex as means of enhancing the effectiveness of the stretch. That is getting the muscle to relax so that we can further stretch the muscle. By adding tension to the GTO by asking our athletes to isometrically contract during PNF stretching we are triggering their bodies into sending this inverse myotatic reflex to inhibit any further contraction of that targeted muscle. This inhibitory reflex is what allows us to stretch the targeted muscle further. Stretching the soft tissue is just one area of the mobility and ROM puzzle. Optimizing mobility leads to proper execution of functional movements, which in turn reduces the likelihood of injury and ultimately improves performance.
Borges, M. O., Medeiros, D. M., Minotto, B. B., Lima, C. S. (2017). Comparison between static stretching and proprioceptive neuromuscular facilitation on hamstring flexibility: systematic review and meta-analysis. European Journal of Physiotherapy, 20. 12-19.
Hindle, K., Whitcomb, T., Briggs, W., & Hong, J. (2012). Proprioceptive Neuromuscular Facilitation (PNF): Its Mechanisms and Effects on Range of Motion and Muscular Function. Journal of Human Kinetics, 31: 105-113.
Maharjan, J., Mallikarjunaiah, H. S. (2015). Proprioceptive neuromuscular facilitation stretching versus static stretching on sprinting performance among collegiate sprinters. International Journal of Physiotherapy, 2, 619-626.
Muscolino, J. E. (2017). Kinesiology: The Skeletal System and Muscle Function. MO, St. Louis. Elsevier
Powers, S. K., & Howley, E. T. (2018). Exercise physiology: Theory and application to fitness and performance. New York, NY: McGraw-Hill Education.